In the fabrication of integrated circuits, semiconductor wafers are usually processed in batches. In the initial stages of processing, wafers are thoroughly cleaned, polished, inspected and tested. It is frequently desirable to mark wafers with indicia indicative of the batch, including eye readable indicia which are useful in retrieving data regarding inspection, testing and manufacture of the integrated circuits.
Some wafer marking has been done by micromachining. In this process, a laser is used to ablate material on the wafer surface, etching a pattern which is controlled by the beam. In another micromachining approach, the laser scans a mask which allows light to pass through in desired locations. The light which passes through may either directly ablate material from the surface, or expose a light sensitive coating which is used to form the desired pattern.
In U.S. Pat. No. 4,594,263 L. Folk et al. disclose a laser marking system wherein an absorptive nickel oxide coating is placed on a surface for enhancing absorption of laser beam energy for ablation of underlying material.
U.S. Pat. No. 4,344,816 to Craighead et al. discloses formation of a surface mask in the form of hill-like structures which are spaced apart on the order of the wavelength of visible light. The hill-like structures are a mask for etching underlying material in an anisotropic etch so that pits are formed having a desired depth.
While formation of eye readable indicia on wafers and similar structures is known, one of the problems which has arisen is that such markings are frequently of low optical contrast and are difficult to read in ambient light.
An object of the invention has been to devise a wafer marking method which may be carried out in a wafer processing operation, yet which is characterized by high optical contrast under ambient light.